Recently, high-power, four-group zoom lenses with high zoom ratios have been increasingly used in handheld TV cameras. Generally, a zoom lens used in an image pickup camera for TV broadcasting requires high performance over the entire range of zoom and over the entire image plane, as well as satisfying certain conditions associated with the use of a solid state image pickup device, such as a CCD. Additionally, there is increasing demand for wide field angles at the wide-angle end along with the high zoom ratio.
As an example, Japanese Laid-Open Patent Application 2001-228396 discloses a high-power zoom lens that responds to the above requirements. This four-group zoom lens includes a focusing lens group, a variator, a compensator, and a relay lens group. The relay lens group, which is the fourth lens group from the object side, includes a front lens subgroup and a rear lens subgroup. The rear lens subgroup is formed of six lens elements, in order from the object side, a first lens element having positive refractive power, second and third lens elements, a fourth lens element that is cemented to a fifth lens element, and a sixth lens element having positive refractive power. The zoom lens described in Japanese Laid-Open Patent Application 2001-228396 uses a large number of lens surfaces in order to provide a high degree of freedom in the design of the zoom lens, achieving a compact design and an achromatic zoom lens by combining lenses with positive and negative refractive powers and different dispersive powers.
In zoom lenses, if a high zoom ratio is to be achieved at a large aperture ratio, it becomes extremely difficult to obtain high optical performance throughout the full range of zoom and the full range of focus. In particular, the refractive power, lens construction and chromatic properties of the lens groups must be properly and strictly prescribed in order to achieve high optical performance without increasing the size or weight of the lens system too much, that is, without excessively increasing the spacings between the lens groups or the number of lens elements.
Currently, there is a desire for optical performance corresponding to high definition broadcast modes. In particular, aberrations at the telephoto end of zooming and aberration variations caused by focusing must be inhibited. If the variation of lateral color or the absolute value of lateral color is not positively inhibited, it is difficult to obtain high resolution.
In the zoom lenses of Japanese Laid-Open Patent Application 2001-228396, a lower refractive power is established and a greater Abbe number is established for the positive lens elements in the second lens component and the third lens component of the rear lens subgroup of the fourth lens group, but it is possible to further reduce lateral color and curvature of field effectively by further adjusting the refractive power and the Abbe number of the positive lens elements. Additionally, in the zoom lenses of the above-mentioned Japanese application, lower refractive power of the first lens element of the rear lens subgroup is also established, but it is possible to further reduce lateral color and curvature of field effectively by further adjusting the refractive power of the first lens element of the rear lens subgroup.